109 midchain branch coverage unchecked

// PINS: branch coverage must be checked wherever a branching tor is invoked,
// not only at a flow head.
//
// `step` declares two branches and no return type, so there is no value to
// bind and `: v` is not a legal spelling for it. Written as a flow HEAD the
// language says so, once per branch:
//
//     ~step(n: 0): v |> ...
//     error[KORU022]: branch 'again' must be handled but no continuation found
//     error[KORU022]: branch 'stop' must be handled but no continuation found
//
// Move the identical mistake one position down the chain — behind any prefix
// invocation — and no wall fires. The binding takes the whole branch union and
// the program reaches emission, where it dies inside Zig's own standard
// library formatting a `step_event.Output` with `{d}`. A diagnostic from
// `std/Io/Writer.zig` is as far from the author's mistake as this compiler
// gets.
//
// The head/mid-chain asymmetry is the whole content of this pin: the check
// exists and is correct, and its reach stops after the first invocation.
//
// 330_074 is the same fault behind a `#loop` head, which is why that test
// emits malformed Zig rather than being rejected — the label is incidental.
// 210_166 is NOT this: its branches are all handled, and it fails in capture
// emission for a legal program.
~import std/io

~tor seed {} -> i64
~proc seed|zig { return 0; }

~tor step { n: i64 }
| again i64
| stop i64

~proc step|zig {
    if (n < 3) return .{ .again = n + 1 };
    return .{ .stop = n };
}

~seed(): n0 |> step(n: n0): v |> std/io:print.ln("bound {{ v:d }}")